Field of the Invention
The present invention relates to a sputtering target for use in forming a Cu—In—Ga—Se compound film (hereinafter referred to as “CIGS film”) for forming a light absorbing layer of a CIGS thin-film solar cell, and a method for producing the same.
Description of the Related Art
In recent years, thin film solar cells made by using a chalcopyrite compound semiconductor have been practically employed. The thin-film solar cell made by using the compound semiconductor has a basic structure in which an Mo electrode layer serving as a positive electrode is formed on a sodalime glass substrate, a light absorbing layer consisting of a CIGS film is formed on the Mo electrode layer, a buffer layer consisting of ZnS, CdS, and the like is formed on the light absorbing layer, and a transparent electrode layer serving as a negative electrode is formed on the buffer layer.
As a method for forming the light absorbing layer described above, a vapor deposition film-forming method is known. Although a light absorbing layer obtained by the method, may exhibit high energy conversion efficiency, the vapor deposition film-forming method attains slow speed for forming a film. Hence, when a film is formed on a substrate having a large area, the uniformity of the in-plane distribution of the film thickness is readily reduced. Thus, a sputtering method for forming a light absorbing layer has been proposed.
As a sputtering method for forming the light absorbing layer, a method has been proposed in which an In film is firstly formed by sputtering using an In target, a Cu—Ga binary alloy film is formed on the In film by sputtering using a Cu—Ga binary alloy target, and a stacked precursor film consisting of the obtained In film and the Cu—Ga binary alloy film is subject to heat treatment in a Selenium atmosphere to thereby form a CIGS film (so called “selenization method”).
Furthermore, in the context of the aforementioned technique, there has been proposed a technique in which the stacked precursor film consisting of the Cu—Ga alloy film and the In film is produced by a sputtering method such that a Cu—Ga alloy layer with high content of Ga, a Cu—Ga alloy layer with low content of Ga, and an In layer are stacked in sequence from the metal backside electrode layer side and the stacked precursor film is then subject to heat treatment in a selenium and/or a sulfur atmosphere to thereby gradually (stepwisely) change the concentration gradient of Ga in the interior of the thin-film light absorbing layer from the interface layer (buffer layer) side to the metal backside electrode layer side so that a thin-film solar cell having a large open circuit voltage can be realized and the peeling-off of the thin-film light absorbing layer from any other layer can be prevented. In this case, there has been proposed that the content of Ga in a Cu—Ga target is in the range of from 1 to 40 at % (see Patent Document 1).
On the other hand, in order to improve the power generation efficiency of a light absorbing layer consisting of a CIGS film, the addition of Na to the light absorbing layer is known as effective means. For example, Non-Patent Document 1 proposes the fact that the content of Na in a CIGS film is typically about 0.1%.
In order to add Na to a light absorbing layer, there has been proposed a method in which a precursor film consisting of In, Cu, and Ga metal elements is formed on a backside electrode layer formed on a substrate, an aqueous solution containing sodium tetraborate, sodium sulfide, and sodium aluminum sulfate is adhered to the precursor film, and then the resulting precursor film is subject to heat treatment in an H2Se gas atmosphere for selenization (see Patent Document 2).